INTERSTAGE COUPLING 



Where the gain of the simple vohage ampHfier is insufficient we achieve the 

 requisite amphfication in several 'stages' by coupUng the output of one valve 

 to the input of the next. Again, when the input swing required by a power 

 amplifying valve in order that the load be fully operated is greater than the 

 available source of signal can provide we interpose a voltage amplifying 

 stage. In both cases the problem arises as to how to couple adjacent valves. 

 There are two broad divisions in the ways in which this can be carried 

 out, depending on the frequency response required. If the response has to 

 extend down to zero frequency the stages have to be 'direct coupled', but 

 if it is possible to allot a lower frequency limit to the desired response the 

 stages may be 'a.c. coupled'. As valves are voltage-operated devices it would 

 be more logical if the term could be amended to 'a.v. coupled', but the notion 

 of the a.c. coupled amplifier is well established and is hkely to remain. As 

 a.c. coupled amplifiers are much easier to design and use it would be fooHsh 

 to use a direct coupled apparatus for, say, action potential recording from a 

 nerve, where a frequency response extending down to 100 c/s would be 

 ample. On the other hand if one is interested in the absolute value of a 

 membrane potential one has no alternative but to use direct coupling*. 



Figure 9.1 



DIRECT COUPLING METHODS 



Climbing amplifier 



By having an HT battery provided with a very large number of taps the 

 grid of the second valve may be connected directly to the anode of the first, 

 in the manner shown in Figure 9.1, which shows two voltage amplifying stages. 



* Except one, a special kind of amplifier called a carrier amplifier, with which we deal in 

 Part IV. 



11 



149 



